The supplementation of vitamin D (vitD) during lactation has been poorly investigated: An expert panel convened by the CDC released a report stating that there are insufficient data to evaluate vitD supplementation requirements during lactation. A CDC study revealed a serious public health matter: 42.4% of African American women in their childbearing years exhibited hypovitaminosis D. Using new setpoints for hypovitaminosis D now renders >90% African American women affected. Nationally, there has been a steady and significant rise in nutritional rickets in breastfed (BF) infants, mainly in the African American population. Health organizations have recommended universal oral vitD supplementation of BF infants to diminish the risk of vitD deficiency in this group. Prolonged supplementation with 25X's the current DRI [10,000 lU/day) for up to 5 mos was shown to be safe in adult men and nonlactating women. Recent data suggests that a minimum of 6,400 lU/day vitDS is essential to prevent or reverse infantile hypovitD, particularly in darker pigmented BF infants. Due to compliance/dosing issues with infants, it is hypothesized that supplementation with higher dose maternal vitD will be safe and as effective as infant oral supplementation in the prevention of vitD deficiency in BF infants and will ameliorate vitD deficiency in lactating women, including women with darker pigmentation. The Aim of this proposal is to determine the effectiveness and safety of maternal and infant vitD supplementation (as a function of ethnicity and latitude) in the prevention of hypovitD in the BF dyad. Mothers from two study sites at different latitudes will be randomized to receive 1 of 3 treatment regimens of vitD3. Mothers, lactating or nonlactating controls will be randomized to either Group A: Standard tx (400 IU D3/d), Group B;2,400 IU D3/day, or Group C: 6,400 IU D3/day. Infants of mothers randomized to Group A will receive 200 IU D3/d (recommended practice) and infants of mothers assigned to Groups B or C will receive placebo. By measuring an array of indicators, calcium homeostasis and skeletal remodeling in the postpartum mother and the breastfeeding infant will be monitored. Through this study, the prevalence of vitD deficiency in the BF dyad and the utility of maternal therapeutic intervention with vitD3 will be assessed. Lay Description: The Aim of this proposal is to determine the effectiveness and safety of maternal and infant vitamin D supplementation as a function of ethnicity and latitude in the prevention of vitamin D deficiency in the breastfeeding mother-infant pair. The findings of this study will generate important new information for health care professionals and policy makers with regard to vitamin D requirements and the potential benefit to both mother and infant.